No MeshRenderer Yes MeshCollider - to fake phantom geometry collision = good idea?

Are there any performance issues to turn off the meshrenderer, but keep the meshcollider for a collision with an "unseen" object?

You still get the performance cost of calculating collisions with the object, but it sounds like you need that, so suck it up.

If you're never going to render the object, then I'd delete the MeshRenderer rather than just disabling it.